The role of the signal sequence in targeting a protein to its intracellular location will be studied by genetic, biochemical, and recombinant DNA methods. By using the yeast Saccharomyces we can take a genetic approach to identifying both elements of structure of the leader itself, and the cellular components with which it interacts. Mutants will be isolated by virtue of being unable to correctly localize a protein synthesized in the cytoplasm and destined for the mitochondria, or by virtue of being inviable due to a defect in protein targeting. The structure of a synthetic mitochondrial targeting sequence will be determined by two- dimensional nuclear magnetic resonance spectroscopy. Mutant presequences, defective in import or cleavage will be studied to correlate alterations in structure with alterations in function. The synthetic presequence will also be used as a probe to try to identify proteins on the mitochondria that act as receptor for targeting proteins to that organelle.